Synergistic wood preservative compositions

ABSTRACT

The invention is directed to synergistic biocidal compositions that combine a copper salt and an organic biocide selected from the group of tribromophenol, its salts and chelates, and sodium-omadine, its salts and chelates. Wood treated with the preservative compositions have improved fungi resistance.

BACKGROUND OF THE INVENTION

The present invention relates generally to wood preservativecompositions, and more particularly to synergistic biocidal compositionsthat combine low cost copper salts and organic biocides to preservewood.

Wood is an important building and construction material and itsimportance and use continues to increase. Examples of its uses includegeneral construction, residential housing, utility poles, crossing arms,fence posts, railroad ties and pilings, etc. For its various uses, woodmust be protected from attack by insects, fungi and other organisms.Insects, such as termites, beetles and carpenter ants, cause extensiveand costly damage to wood structures. Wood-attacking fungi, such asbrown-rot and white-rot, will structurally weaken wood by damaging thecellulose and lignin. If left unprotected or unpreserved, wood willdecay and deteriorate within a period of a few months to a few years,depending upon climate and soil conditions. Wood objects such as utilitypoles and timbers will deteriorate rapidly below ground level and willrequire frequent replacement if not properly and adequately preserved bychemical preservative treatment.

Toxicity to various wood-attacking organisms is an essential requirementof wood preservatives. The preservative treatments for wood musteffectively repel attacking organisms for an extended period, i.e.,provide long lasting protection. In addition, wood preservatives shouldhave favorable wood penetrating properties, should not corrode metalsand should be safe to handle by those treating the wood and using thefurnished wood product.

Through the years, a number of different compounds have been used forpreserving wood. Among these compounds include creosote, heavy metalsalts, heavy oils and tars, pitch and various organic chemicals. Each ofthese compounds has its advantages and drawbacks.

For instance, creosote, as well as other heavy oils, tars and pitchtreatments, has a strong odor and leaves the surface greasy and oily tothe touch making the treated wood more difficult to handle. Halogenatedphenols are also widely used as commercial wood preservatives becausethey have good fungicidal and insecticidal properties. The halogenatedphenols are dissolved in hydrocarbon solvents, such as medium aromaticoils, volatile petroleum solvents (propane), light solvents (mineralspirits), or in a chlorinated hydrocarbon solvent-inhibited grade ofmethylene chloride.

Pentachlorophenol is a commonly used phenol preservative due to itseffectiveness, relative ease of application and durability. However,recent environmental concerns are expected to limit the use ofpentachlorophenol in the future. Copper and other heavy metals are goodbiocides. However, copper is not effective as a wood preservative sinceit does not control Postia sp. and other fungi which are coppertolerant.

Wood preservatives can be applied to wood as solutions, emulsions,pastes or dispersions in liquid hydrocarbons and/or aqueous systems.Water-borne preservatives are usually less expensive. In manyapplications, the use of water-borne preservatives are preferred,because of the odors, flammability and often toxic nature of liquidhydrocarbon solvents.

The protection afforded by the application of any wood preservative isdependent upon the depth and uniformity of the penetration into the woodor timber. The treatment of wood material usually comprises soaking orimpregnating the wood with the fluid-borne treating chemicals. Anothertechnique that is practiced in a number of variants is the full-cellprocess, where the pores of the wood material are partially evacuatedbefore impregnation in order to increase the fluid retained therein, orthe empty-cell process which is similar to the full-cell process exceptthat the preliminary step of evacuation is omitted.

The most common commercial procedures for impregnating wood involvecontacting the wood with the preservative under a relatively highpressure, for example, 50-200 psi (pounds per square inch) for a periodof time, such as from one-half hour to twenty four hours. The processingmay also require relatively high temperatures in the range from about75° C. to about 105° to 110° C.

Upon contact and penetration into wood materials and during the dryingprocess, the preservative in the wood preferably becomes fixed andessentially non-leachable. In various formulations, water in widelyvarying percentages is used as the main solvent for the treatingsolution.

In the course of treating wood with different biocides, certain insectsand fungi are found not to be repelled by particular chemicaltreatments. In order to protect against a broader range of such insectsand fungi, combinations of preservatives are sometimes used. Somecombinations of preservatives have been found to have a greatereffective than the sum of their individual components, and areconsidered synergistic.

An object of the present invention is to provide copper salt and organicpreservative compositions that have synergistic effects in treatingwood.

Another object of the present invention is to provide combinations ofpreservatives that are highly effective and less costly to use.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention.

SUMMARY OF THE INVENTION

The present invention is directed to a synergistic biocidal compositioncomprising a copper salt with an organic biocide selected from the groupconsisting of tribromophenol (TBP), its salts and chelates, and sodiumomadine, its salts and chelates. Wood treated with the preservativecompositions of this invention are characterized by improved fungiresistance. It is found that the copper salts, such as copper sulfate,copper acetate or copper carbonate, in these treating systems provideadditional fixation and preservative properties.

According to the present invention, it has been found that the additionof TBP (2,4,6-tribromophenol) to a copper salt preservative will controlthe copper tolerant fungi and will act synergistically to preserve wood.The combination of sodium omadine (sodium pyrithione, C₆ H₄ NOSNa or2-pyridenthiol-1-oxide, sodium salt) and a copper salt preservative alsoproduces similar synergistic results.

The present combinations of preservatives have been found to have agreater effectiveness than the sum of their individual components at agiven concentration. The present invention will also protect against abroader range of insects and fungi. In addition, the presentformulations eliminate the need for arsenic and chromium in thepreservative formulation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in terms of certain specificembodiments and examples.

As used herein, the term "synergism" is intended to include both anincreased spectrum of activity (i.e., greater activity against a largernumber of microorganisms), and/or increased activity (i.e., greateractivity against specific organisms than that predicted by use of eitheragent alone). Both of these factors play an important role in theeconomy and commercial acceptance of a biocide. Increased activity ofthe combination permits the use of smaller amounts of biocides toachieve satisfactory control with resulting economies. Increasing thespectrum of activity permits wider use of the biocide in environmentscontaining many and diverse microorganisms which must be controlled.

The compositions used in the method of the present invention may alsocontain other additives depending on the intended use for thecomposition. When the compositions of the present invention are used inpreserving wood, other additives may be included which impart desirableproperties to the treated wood. For example, the compositions maycontain anti-foam agents, surfactants, antioxidants, flame retardantcompositions, coloring agents, insecticides, deodorants, mobicides, woodstabilizing agents, etc. The amount of such optional additives includedin the composition of the present invention may vary over a wide range,although amounts of about 0.1 to 10.0 percent of these compositions aregenerally satisfactory.

Although copper salt and sodium omadine, individually, are both watersoluble, combining these two compounds results in a reaction that formsa water-insoluble complex that precipitates in water. A suitable organicsolvent can be used to dissolve the complex and form the presentpreservative composition for treating wood. Alternatively, the complexcan be emulsified in water by using a small amount of organic solventand a suitable surfactant. First, the complex is dissolved in theorganic solvent. Then the surfactant is added and an emulsion is formedin water by thoroughly stirring the mixture. Various known combinationsof surfactants and solvents can be used to produce stable emulsions fortreating wood. The preparation and treatment using a copper-sodiumomadine emulsion is the least expensive method.

An application method called a "dual water-borne" treatment can be used.A water-borne system has the advantage of being low cost,environmentally less hazardous and non-flammable, as compared with anorganic solvent-borne system. Example 1 below gives the formulations fora dual water-borne treatment based on parts by total weight:

EXAMPLE 1

    ______________________________________                                                       Broad     Preferred                                                           Range     Range                                                ______________________________________                                        Formula A                                                                     Copper           0.25    to 10.0 0.5   to 3.0                                 Water            90      to 99   97    to 99                                  Formula B                                                                     Sodium Omadine   0.1     to 2.0  0.1   to 1                                   Water            98      to 99   99                                           Ratio of A to B  20:1    to 1:2  10:1  to 2:1                                 (Copper to Sodium Omadine)                                                    ______________________________________                                    

The dual water-borne treatment is carried out by the sequentialapplication of an aqueous sodium omadine solution (Formula B) followedby an aqueous copper salt solution (Formula A), or vice verse. InFormula A, the copper salt is dissolved in preferably an ammoniacalsolvent, and in Formula B, the sodium omadine is prepared in water asthe solvent. Depending on how a wood is to be used, the wood may bedried after application of the first formula and then treated with thesecond formula. A well known method called the "empty cell process" canalso be used, wherein the first formula is applied in such a manner soas to permit an additional water-borne treatment to occur. The coppersalt and sodium omadine react to form the water-insoluble complex insidethe treated wood.

Alternatively, the copper:sodium omadine complex, in the ratios shownabove, can be formed in water and dissolved in an organic solvent.Alternatively, the two copper salt and organic biocide can be addeddirectly to a suitable organic solvent. The preservative compositionscan also be formulated as an emulsion, as described above, and thetreatment completed as single step. The oil-borne systems also have theadvantage of allowing the wood to be treated in a single step process.Regardless of the application method used, by forming a water-insolublecomplex, the present preservative compositions provide non-leacheablebiocides for treating wood products that are fixed in the wood and canbe effective over a long period of time.

The copper and TBP compositions can also be applied using a dualwater-borne treatment when a water soluble form of TBP, a phenolatesalt, is used and the copper salt is prepared as an aqueous solution.When the TBP is prepared with an organic solvent, a two-step treatmentcan also be used. Copper will form a complex with TBP. This complex isdissolved in an organic solvent and the wood is treated in a mannersimilar to that described above for copper-sodium omadine.

Example 2 below gives the formulation for a two-step treatment using anaqueous copper solution and TBP in an organic solvent based on parts bytotal weight:

EXAMPLE 2

    ______________________________________                                                     Broad          Preferred                                                      Range          Range                                             ______________________________________                                        Formula A                                                                     Copper         0.25   to 10.0     0.5  to 3.0                                 Water Solvent  90     to 99       97   to 99                                  Formula B                                                                     TBP            0.25   to 10.0     0.5  to 3.0                                 Organic Solvent                                                                              90     to 99       97   to 99                                  Ratio of A to B                                                                              20:1   to 1:20     8:1  to 1:8                                 (Copper to TBP)                                                               ______________________________________                                    

Again, the wood can be dried after treatment with the first formula,then followed with treatment with the second formula, or the empty cellprocess may be used. Alternatively, copper and TBP can be formulatedtogether, such as in an emulsion, and the treatment completed in onestep.

The contact between the wood and the mixtures of the present inventioncan be effected by numerous methods, including brushing, spraying,painting, pressure treating, immersing, etc. Preferably, contact betweenthe wood and the mixtures of the present invention are effective byimmersing the wood in the mixture for a period of time which issufficient to obtain the desired results.

Treating solutions in commercial volumes are typically in the range of:

5,000 to 50,000 gal.

10,000 to 25,000 gal. (preferred)

Contact times with wood may vary from:

0.5 hours to 24 hours

0.5 hours to 3 hours (preferred)

The compositions of the present invention can be used for preserving awide variety of wood types. Examples of wood species which can betreated with the compositions of the invention include Southern YellowPine, Western Red Cedar, Douglas Fir, Inland Fir, Spruce, Hemlock, SugarMaple, Ash, Walnut, Cherry, White Pine, Red Pine, Birch, Red Oak, WhiteOak, Elm, Hickory, Linden, Beech, Sycamore, etc.

The compositions of the present invention will also contain at least onesolvent/dilutant. The solvent/dilutant may be water or a liquidhydrocarbon. Suitable hydrocarbon liquids include aromatic and aliphatichydrocarbon solvents such as petroleum hydrocarbon solvents, aromatichydrocarbons, aromatized petroleum distillates, and mixtures orpetroleum hydrocarbon solvents and aromatic hydrocarbon solvents.Examples of useful solvents include xylene, toluene, naphtha, lightmineral oil, etc.

The preferred copper salt of the present invention is copper carbonate.However, it is within the scope of the invention to use other coppersalts, such as copper sulfate, copper chloride, copper acetate, copperoxide, copper hydroxide, etc. The copper compound is dissolved inpreferably an aqueous ammoniacal solution which has a preferred pH rangeof about 10.8 to 11.0.

The test results below provide determinations of the minimum amount ofpreservative that is effective in preventing decay of selected speciesof wood by selected fungi under laboratory conditions and show thesynergistic effects of the combinations.

The soil block and agar plate test methods were used to examine thebiocidal compositions of the present invention. These commonly usedmethods are described in Wood Deterioration And It's Prevention ByPreservative Treatments, Vol.2., edited by D. Nicholas, SyracuseUniversity Press (1973), which is incorporated by reference herein. Thesoil block method involves impregnating small wood blocks to one or moredifferent retention levels with the biocidal composition. The testblocks were placed on an untreated wood wafer which rested on soil thathad been inoculated with a specific wood decay fungus.

Conditioned blocks of wood were impregnated with solutions ofpreservatives in water or suitable organic solvent to form one or moreseries of retention levels of the preservative. After a period ofconditioning, the impregnated blocks were exposed to a recognizedbrown-rot wood-destroying fungus. The effectiveness of the biocide isusually determined by measurement of weight loss of the treated woodblocks after a specific incubation period.

When using soil substrates, the soil should have a water-holdingcapacity between 20 and 40 percent and a pH between 5.0 and 8.0. Afterbreaking up all clumps, the soil was mixed and screened through a U.S.No. 6 sieve and stored in a large covered container. The soil should notbe so wet that when sifted the particles again stick together. Theoven-dried weight of 118 ml of sifted soil should not be less than 90grams. The soil is preferably obtained from forested rather thanagricultural sites. If the pH of the soil is below 5.0, the soil shouldbe amended with hydrated lime to adjust the pH to between 5.0 and 8.0.

The agar plate method involves the preparation of agar nutrientsolutions containing various biocide concentrations. The agar solutionswere poured into petri dishes and inoculated with a plug of agar whichcontained an actively growing fungus culture. The diameter of the fungalculture was measured after a set incubation time. The radial myceliumgrowth of the fungus on the agar plates which contain biocide was thennormalized to the growth on the untreated (control) agar plate. Thelowest biocide concentration (parts per million, or ppm) which totallyinhibited fungal growth (minimal inhibitory concentration or MIC) wasdetermined. In addition, the concentration of biocide necessary toinhibit the fungal mycelium growth by 50% (IC₅₀, ppm, or inhibitoryconcentration, 50%) was also determined.

The advantages of the agar plate test are that it is rapid (1-2 weeks)and requires a minimal amount of biocide (0.05-0.5 grams). In contrast,the soil block test requires 12-18 weeks and at least one gram ofbiocide. However, the longer test period of the soil block tests can beused to identify preservative compositions which may be degraded by thefungi.

For the agar plate tests, the biocide concentrations necessary toinhibit the wood decaying fungi are extremely low, and thus the resultscannot be related to concentrations actually needed to protect wood inthe field, whereas the soil block tests provides an approximation of thebiocide concentration required to protect wood in service. The agarplate test gives results which can often be reproduced in subsequentperformances of the test, while subsequent soil block tests do notnecessarily give reproducible results.

The wood-destroying fungi listed below were used in the following tests.

P. placenta=Postia placenta, American Type Culture Collection (ATCC)#11538, a brown-rot fungi which is copper tolerant.

G. trabeum=Gloeophyllum trabeum, ATCC #11539, brown-rotter.

T. versicolor=Trametes versicolor, ATCC #12679, white-rotter.

C. globosum=Chaetomium globosum, ATCC #6205, soft-rotter.

I. lacteus=Irpex lacteus, ATCC #11245, white-rotter.

Table 1 below gives the agar plate results with copper and sodiumomadine using four wood destroying fungi. The activity values, IC₅₀ andMIC (ppm), for the copper (copper(II) chloride or CuCl₂) alone andsodium omadine alone are given, along with the activity values for acombination of three parts copper and one part sodium omadine. Thecopper solution was tested at 10, 30, 100, 300, and 1000 ppm levels. Thecopper:sodium omadine (3:1) combination and sodium omadine alone weretested at 3, 10, 30, 100, and 300 ppm levels. As shown in Table 1, thecombination of copper:sodium omadine (3:1) results in greaterbioactivity values (lower IC₅₀ and/or MIC) than either copper or sodiumomadine individually.

Synergism is exhibited by the combination having greater activity thaneach individual component alone when tested at the same concentration.In addition, the synergistic factor (SF) can be calculated. If SF isequal or greater than 1.5, then synergism is indicated (see U. Gisi etal. Synergistic Interactions of Fungicides with Different Modes ofAction, Trans. Br. Mycol. Soc., 85 (2), at 299-306 (1985)). All SFvalues calculated using the data in Table 1 are much greater than 1.5.

                                      TABLE 1                                     __________________________________________________________________________    AGAR PLATE ACTIVITY                                                           VALUES FOR COPPER AND SODIUM OMANDINE.                                        Biocide                                                                       (Solvent)    Copper  Omadine                                                                              Copper:Omadine                                    (Biocide Concentration                                                                     (Water) (Water)                                                                              (Water)                                           Range)       (10-1,000 ppm)                                                                        (3-300 ppm)                                                                          (3-300 ppm)                                       __________________________________________________________________________    I. lacteus; IC.sub.50 /MIC                                                                 53/100  7/300  2/10                                              T. versicolor; IC.sub.50 /MIC                                                              95/100  8/100  3/10                                              G. trabeum; IC.sub.50 /MIC                                                                 67/300  32.sup.= /100 .sup.                                                                  4/30                                              P. placenta, IC.sub.50 /MIC                                                                167/1000 (300*)                                                                        5/30  2/10                                              __________________________________________________________________________     *Very close to MIC.                                                           IC.sub.50 Concentration (parts per million) at which the relative fungal      growth is inhibited by 50@.                                                   MIC Concentration (parts per million) at which the fungal growth is           totally inhibited.                                                            .sup.= Mutated at 3 ppm.                                                 

Additional agar plate tests were carried out using copper solutions(copper(II) chloride), sodium omadine solution, and copper:sodiumomadine compositions of ratios of 1:1, 3:1, and 6:1. Water was used asthe solvent for all biocides, and the levels tested were 1, 2, 5, 15,40, and 100 ppms. The four wood destroying fungi used included twobrown-rotters, one of which is copper tolerant, one white-rotter and onesoft-rotter. The results are shown in Table 2. The 1:1 and 3:1preservative compositions proved to be synergistic for all four fungibased on reduced MIC values as compared to the individual biocide's MICvalues and/or SF values. In addition, for the 6:1 combination, the SFvalues for three of the four fungi are greater than 1.5 (P. placenta, T.versicolor, and C. globosum) and are thus considered to be synergistic.

                                      TABLE 2                                     __________________________________________________________________________    AGAR PLATE ACTIVITY                                                           VALUES OF COPPER AND SODIUM OMADINE.                                          Biocides                                                                      Copper:Sodium  Fungi Activity (IC.sub.50 /MIC, ppm)                           Omadine Solvent                                                                              P. placenta                                                                          G. trabeum                                                                          T. versicolor                                                                        C. globosum                                __________________________________________________________________________    1:0     Water  >100/>100                                                                            70/>100                                                                             ˜159/>100                                                                      53/>100                                    0:1     Water  4/40   ˜2/>100                                                                       8/>100*                                                                              3/100                                      1:1     Water:Water                                                                          1/15   4/40.sup.(15*)                                                                      5/100.sup.(40*)                                                                      5/40.sup.(15*)                             3:1     Water:Water                                                                          5/40   5/40  6/40   8/40                                       6:1     Water:Water                                                                          13/40  11/>100                                                                             8/>100*                                                                              3/>100*                                    __________________________________________________________________________     *Very close to MIC                                                       

Additional tests were conducted using the soil block test and thebrown-rot fungus G. trabeum. Wood blocks were treated using: (1) anaqueous copper salt solution alone (ammoniacal copper solutions); (2) anaqueous sodium omadine solution alone; and (3) different combinations ofcopper (ammoniacal copper solutions) and sodium omadine. The averageretentions, in pounds per cubic foot (pcf), are listed along with theaverage percent weight loss for the wood blocks in Table 3 below. Theresults showed the individual biocides at treatment levels of about 0.09pcf did not offer full protection against the wood destroyingfungi-treatments with copper at 0.091 pcf resulting in a 6.0% weightloss and sodium omadine at 0,088 pcf resulting in a 7.8% weight loss.

The combination of copper and sodium omadine, however, at the lowerretention level of 0.07 pcf gave essentially complete protection atcompositions of 1:1 (0.14% weight loss) and 3:1 (1.21% weight loss).Consequently, synergism is evidenced by the combination of copper andsodium omadine having a greater efficiency at a lower level than eithercopper or sodium omadine when used alone.

                  TABLE 3                                                         ______________________________________                                        AVERAGE PERCENT WEIGHT LOSS OF                                                TREATED SOUTHERN PINE BLOCKS EXPOSED TO A                                     BROWN-ROT FUNGUS (G. TRABEUM) FOR 8 WEEKS.                                    ______________________________________                                        INDIVIDUAL COMPONENTS                                                         Compounds                                                                              Avg. Retention (pcf)                                                                          Avg. Weight Loss (%)                                 ______________________________________                                        Copper   0.091           6.0                                                           0.187           0.7                                                           0.392           1.0                                                  Omadine  0.036           19.1                                                          0.088           7.8                                                           0.182           0.1                                                  ______________________________________                                        COMBINED COMPONENTS                                                                       Avg. Retention                                                                             Total    Avg. Weight                                             Copper/Omadine                                                                             Retention                                                                              Loss                                        Compounds   (pcf)        (pcf)    (%)                                         ______________________________________                                        Copper & Omadine                                                                          0.035/0.035  0.07     0.14                                                    0.070/0.035  0.11     0.25                                                    0.054/0.018  0.07     1.21                                                    0.102/0.034  0.13     0.62                                        Untreated Controls                                                                        0/0          0        46.2                                        ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        AVERAGE PERCENT WEIGHT LOSS OF                                                TREATED SOUTHERN PINE BLOCKS EXPOSED TO A                                     BROWN-ROT FUNGUS (P. PLACENTA) FOR 8 WEEKS.                                   ______________________________________                                        INDIVIDUAL COMPONENTS                                                                  Avg. Retention (pcf)                                                 Compounds                                                                              Weight loss     Avg. Weight Loss (%)                                 ______________________________________                                        Copper   0.188           38.6                                                          0.385           21.6                                                          0.778           20.1                                                 Omadine  0.037           13.1                                                          0.094           2.9                                                  ______________________________________                                        COMBINED COMPONENTS                                                                   Avg. Retention                                                                              Total    Avg. Weight                                            Copper/Omadine                                                                              Retention                                                                              Loss                                           Compounds                                                                             (pcf)         (pcf)    (%)                                            ______________________________________                                        Copper &                                                                              0.036/0.036   0.072    4.7                                            Omadine 0.073/0.037   0.110    0.9                                                    0.110/0.037   0.147    1.5                                            Untreated                                                                             0/0           0        26.0                                           Controls                                                                      ______________________________________                                    

Additional soil block tests were conducted using a copper-tolerantbrown-rot fungus, P. placenta, and the results are provided in Table 4above. As can be seen, treatment with only an amnioniacal copper saltsolution provided no protection, even at the highest treatment level of0.778 pcf (20.1% average weight loss). Wood treated with only sodiumomadine at 0.037 pcf yielded a 13.1% average weight loss. When copperand sodium omadine were combined, however, increased activity wasobserved. For example, when a treatment of 0.073 pcf of copper and 0.037pcf of sodium omadine was used, the average % weight loss was 0.9%, andwhen a treatment of 0.110 pcf of copper and 0.037 pcf of sodium omadinewas used, the average % weight loss was 1.5%.

Table 5 below gives the agar activity values for copper alone, TBP aloneand a mixture of copper and TBP (1:1). The levels tested were 5, 10, 25,60, 150 and 300 ppm. The copper-tolerant brown-rot fungus P. placentaand the white-rot fungus T. versicolor were used. The SF values areabove 1.5 for both fungi.

                  TABLE 5                                                         ______________________________________                                        AGAR PLATE                                                                    ACTIVITY VALUES FOR COPPER AND TBP.                                                                  Fungi Activity                                         Biocides               IC.sub.50 /MIC, ppm)                                   Copper:TBP                                                                              Solvent      P. placenta                                                                              T. versicolor                               ______________________________________                                        1:0       Water        165/>300*  77/>300                                     0:1       Toluene      7/25       16/60                                       1:1       Water:Toluene                                                                              9/25       12/60                                       ______________________________________                                         *Indicates a MIC in which the fungus is almost totally inhibited.        

The copper and TBP compositions were further tested using the soil blocktest with a copper tolerant fungus P. placenta, and the results areshown in Table 6 below. The wood blocks treated with copper aloneresulted in weight loss even at the highest level of treatment, 0.837pcf. The samples treated with TBP alone resulted in weight loss at 0.157pcf (6.7% average weight loss). The preservative compositions of copperand TBP (0.085 pcf of copper and 0.085 pcf of TBP) resulted in only 5.2%average weight loss, and the samples treated with 0.154 pcf of copperand 0.077 pcf of TBP resulted in only 1.4% average weight loss.

                  TABLE 6                                                         ______________________________________                                        AVERAGE PERCENT                                                               WEIGHT LOSS OF TREATED SOUTHERN                                               PINE BLOCKS EXPOSED TO A COPPER TOLERANT                                      BROWN-ROT FUNGUS (P. PLACENTA) FOR 8 WEEKS.                                   ______________________________________                                        INDIVIDUAL COMPONENTS                                                         Compounds                                                                              Avg. Retention (pcf)                                                                          Avg. Weight Loss (%)                                 ______________________________________                                        Copper   0.099           45.4                                                          0.202           51.8                                                          0.412           42.1                                                          0.837           29.3                                                 TBP      0.081           33.6                                                          0.157           6.7                                                           0.337           0.6                                                           0.642           0.4                                                  ______________________________________                                        COMBINED COMPONENTS                                                                     Avg. Ret., pcf                                                                            Total      Avg.                                         Compounds Copper:TBP  Retention, pcf                                                                           Wt. Loss (%)                                 ______________________________________                                        Copper:TBP                                                                              0.085:0.085 0.170      5.2                                                    0.165:0.165 0.330      0.1                                                    0.154:0.077 0.231      1.4                                                    0.099:0.197 0.296      0.8                                          Control   --          --         38.2                                         ______________________________________                                    

The foregoing soil block tests were conducted using a conditioning roommaintained at temperatures of approximately 20° and 30° C. and anaverage relative humidity between 25 and 75%. The selected temperaturewas not varied by more than ±2° C. (±4° F.). The incubation room wasmaintained at a selected temperature between 25° and 27° C. (77° and81°) and an average relative humidity between 65 and 75%. The selectedtemperature was varied by more than ±2° C. (±4°) and the selectedhumidity not more than ±5%. Lights remained off during decay testsexcept when needed for period inspection.

The wood used in the test came from newly cut boards, nominally 1 inchthick, that are immediately kiln-dried without anti-stain treatment toprovide chemically free wood that has had minimum opportunity for fungusinfection or deterioration before use in the soil-block.

The treating solutions of the preservative must be carefully prepared inappropriate concentrations. The concentration of the preservatives to beused should provide a uniform distribution of preservative at retentionslow enough to permit fungus attack and to determine the threshold valuesfor the various test fungi employed.

Finally, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation shown and described, andaccordingly all suitable modification and equivalents may fall withinthe scope of the invention.

The present invention has been described in terms of a preferredembodiment and is considered as illustrative only of the principles ofthe invention. The invention, however, is not limited to the embodimentdepicted and described. PG,20

What is claimed is:
 1. A biocidal composition for preserving wood havingimproved efficacy comprising a combination of:(a) a copper salt; and (b)an organic biocide selected from the group consisting of tribromophenol,the salts and chelates of tribromophenol, and sodium pyrithione, and thechelates thereof, wherein the combination forms a complex that issubstantially non-leachable from wood and wherein the weight ratio ofcopper to tribromophenol is in the range of about 20:1 to 1:20, and theweight ratio of copper to sodium pyrithione is about 20:1 to 1:2.
 2. Acomposition according to claim 1 wherein the copper salt is dissolved inan aqueous ammoniacal solution having a pH range of about 10.8 to 11.0.3. A composition according to claim 1 wherein the combination of coppersalt and organic biocide is prepared as an emulsion.
 4. A compositionaccording to claim 1 wherein the weight ratio of copper totribromophenol is in the range of about 8:1 to 1:8.
 5. A compositionaccording to claim 1 wherein the weight ratio of copper to sodiumpyrithione is in the range of about 10:1 to 2:1.
 6. A compositionaccording to claim 1 wherein said copper salt is copper carbonate.
 7. Acomposition according to claim 1 further containing additives comprisingone or more of anti-foam agents, surfactants, anti-oxidants, flameretardants, coloring agents, insecticides, deodorants, malicides andstabilizing agents in amount in the range of about 0.1 to 10.0 percentby weight.